Disentangling the Relative Impacts of Temperature and Vapor Pressure Deficit on Tropical Forest Photosynthesis

Claire Zarakas, University of Washington

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Tropical forest photosynthesis can decline at high temperatures due to 1) biochemical responses to increasing temperature and 2) stomatal responses to increasing vapor pressure deficit (VPD), which is associated with increasing temperature. It is challenging to disentangle the influence of these two mechanisms on gross primary production (GPP), because temperature and VPD are tightly correlated in tropical forests. Nonetheless, quantifying the relative strength of these two mechanisms is essential for understanding how tropical GPP will respond to climate change, because increasing atmospheric CO2 concentration may partially offset VPD-driven stomatal responses, but is not expected to mitigate the effects of temperature-driven biochemical responses. We used a demographic ecosystem model - the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) - to quantify how functional traits and physiological process assumptions (e.g. about photosynthetic temperature acclimation and plant hydraulics) influence the relative strength of modeled temperature vs. VPD effects on light-saturated tropical forest GPP. We simulated sites spanning different humidity regimes - including Amazon forest sites and the experimental Biosphere 2 forest - to test which process and functional trait assumptions best capture the GPP responses to VPD vs. temperature identified in observational studies. Next, by simulating idealized climate change scenarios, we quantified the divergence in GPP predictions under model configurations with stronger indirect (VPD) effects compared to stronger direct temperature effects. Our findings underscore the importance of distinguishing between direct temperature and indirect VPD effects, and demonstrate that the relative strength of temperature vs. VPD effects in models is highly sensitive to plant functional parameters and structural assumptions about photosynthetic temperature acclimation and plant hydraulics.

Abstract Author(s): Claire Zaraka, Abigail Swann, Charles Koven, Marielle Smith